What is best way to carve the foam for running long 6" or 8" ducts under an insuldeck floor? Using a hot wire may be very tedious so I am thinking to use an electric knife such as this http://www.amazon.com/dp/B001NXC7TA/ref=wl_it_dp_o_pC_nS_ttl?_encoding=UTF8&colid=GBGGPY66BYDR&coliid=I2P9WG2YTIZ120 Do you have a better way of doing it or How is it normally done?

Not sure if the link I posted above works so I'm adding a picture:
http://ecx.images-amazon.com/images/I/61TIlP8jZoL._SL1484_.jpg

It's a Rapala Rechargeable Cordless Electric Fillet Knife.

i used a sawzall with a 4" or 6" long wood cutting blade.
then i cut a grid of 2" vertical squares.
once i removed the first few pieces, the rest broke out easy by hitting them at their base with a small hammer.
if you are removing the foam all the way to the concrete slab, cut and then chisel out the foam by scraping the tool along the concrete.
insuldeckflorida at aol dot com if you need more help.

I haven't tried an electric carving knife--worth trying. But I've found a utility knife of the type with segmented snap-off blades, extended so you have a long blade, works really well. The fact that they are thin really helps it slip through without tearing.

I don't like using a saw, unless I'm working in a closed space where I can vacuum up the dust. I don't think it's a good idea to spread the dust around outside.